In conventional deep fat fryers, there has been utilized heating units of the direct-fired type, which are adapted to heat a cooking oil in the fryer by direct firing of combustible gases into heating tubes running through the bath from jet burner tubes associated with the fryer. The direct-fired heating units and the heating tubes thereof are conventionally positioned in parallel relationship running from side-to-side in the deep fat reservoir, with the jet burner tubes positioned at an open end of each of the tubes to introduce a combustible gas which is made to burn throughout the length of each of the heating tubes to heat the oil to a selected temperature. It should be recognized that with the direct-fired type of heating unit, uniformity of temperature within each of the heating tubes is difficult to maintain, resulting in non-uniformity of the heated cooking oil within the deep fat fryer. The cooking oil will become hotter adjacent the flame and of the burner tubes, and becomes progressively relatively cooler toward the opposite or flue end of the burner tube. The non-uniform heating of the frying oil will result in non-uniform frying of the food products within the fryer, and has also been found to result in turbulence within the fat reservoir which is undesirable.
As an alternative, indirect type of heating units have been developed, which comprise radiator-type units submerged in the cooking oil of the fryer. The radiator-type units normally include a plurality of tubing loops which carry a hot, pressurized, circulating heating fluid which is heated at a source remote from the fryer. In this manner, the heating fluid is brought to a predetermined temperature, and thereby raises the radiator-type heating unit submerged in the deep fat bath to a temperature close to that predetermined temperature. The indirect type of heating unit has been found to yield better uniformity of cooking oil temperatures within the fryer, but has resulted in various additional problems. As such deep fat fryers are typically used for frying of foods such as breaded poultry, breaded fish and shrimp, meat in a batter or breaded form, potato chips, tortilla chips, and the like, from which food particles such as batter, bread, meat, and like debris is loosened, such debris falls through the hot fat bath onto the array of heating units within the bath. Such food products will tend to adhere to the array of heating units in both the direct and indirect types of heating units, and cause significant problems in cleaning and maintaining the fryer apparatus.
Additionally, the indirect type of heating unit has also been found to not provide a substantially uniform temperature distribution within the hot oil bath, as the heating fluid introduced to the array of tubing loops associated with the radiator-type units submerged within the deep fat bath will cool as it is circulated therethrough. Alternatively, a circulation system associated with the radiator-type units will experience a large pressure drop from the infeed end of the tubing loops to the outlet end thereof. The pumping requirements associated with such systems must therefore be of quite large capacity to ensure that the heating fluid is adequately circulated through the plurality of tubing loops. Such indirect type of heating systems have also been found to not provide efficient heat transfer from the heating fluid to the surrounding oil bath, and the heating fluid directed into the radiator-type units must therefore be raised to a significantly higher temperature to ensure heating of the hot oil bath to a desired temperature.